Method and apparatus for routing traffic in a circuit-switched network

ABSTRACT

A method and an apparatus for routing traffic in a circuit-switched network, comprising offering a call between between an origin node and a destination node to a preferred route between said nodes, and if said preferred route is not available, offering an alternative route via an intermediate node, and for links between two nodes, setting a first trunk reservation threshold for reserving a certain number of circuits for direct calls along said links between said two nodes, and setting a second trunk reservation threshold for calls between nodes connecting a second link of said alternative route.

This application is the national phase under 35 U.S.C. §371 of prior PCTInternational Application No., PCT/SE96/00862 which has an Internationalfiling date of Jun. 28, 1996 which designated the United States ofAmerica, the entire contents of which are hereby incorporated byreference.

The present invention relates to telecommunications networks andmethods. In particular it relates to routing of traffic in acircuit-switched network with full or multiple interconnection.

DESCRIPTION OF RELATED ART

In a telecommunications netword traffic is normally routed from anorigin to a destination along a pre-assigned direct route. With state ofthe art switching technologies it is possible to divert overflow trafficon overloaded direct routes along alternative paths and computercontrolled exchanges, and common channel signalling allows for anincreased flexibility in the implementation of routing methods.

In a fully interconnected network each node is connected to every othernode by a link comprising a number of two-way circuits. Alternatively,each node pair must be connected via one or several intermediate nodes.Alternative routes are, however, in practice usually confined totwo-link paths between an origin node and a destination node. Callsbetween a pair of nodes are first routed via the direct link between thenodes. If the direct link is blocked, the overflowing call is routed onan alternative two-link path. In order to prevent overflowing calls onan alternate two-link path from blocking subsequent direct calls onthese links, circuit reservation is applied by setting a trunkreservation threshold for each link. Traffic routed on a direct link canuse any idle circuit, whereas overflowing calls are accepted on a linkonly when the number of idle circuits exceeds the trunk reservationthreshold.

The development of routing has gone from hierarchical routing methodswith fixed alternative paths between nodes arranged in a pre-definedhierarchy to dynamic non-hierarchical routing methods, whereinalternative routes are chosen in a more flexible fashion. Dynamicmethods has proved to be important tools to adapt networks to changingtraffic demands, shifts in traffic patterns and network failures, aswell as to even out excessive idle capacity in some parts of the networkand overload in other parts.

Dynamic routing methods are basically divided into time dependentmethods and state dependent methods. Time dependent methods are focusedon systematic variations in the traffic load of a network, and pathsbetween nodes are redefined at fixed times. This method is directed tothe handling of forecasted traffic fluctuations, but gives a poorprotection against unplanned traffic variations. State dependent methodsuse information about the current state of a network to decide analternative path and adapt well to unplanned traffic changes. It is alsoknown to use a combination of time dependent and state dependentmethods.

State dependent methods are divided into centralized, distributed andisolated methods, depending on how the information about the currentstate of a network is handled. In networks employing centralized methodsa central network processor has information about the states of alllinks of a network, and when an alternative path is needed the leastloaded path is recommended. In networks adapted to distributed methodsan node has information about the state of its own links and is alsoable to obtain information about specific links from other nodes. Innetworks using isolated methods the nodes have information only abouttheir own links.

A centralized dynamic routing method, which in this description iscalled centralized least loaded path routing or CLLPR is disclosed inthe document Regnier J., Cameron W. H., State-Dependent Dynamic TrafficManagement for Telephone Networks, IEEE Communications Magazine,September 1987. The CLLPR method makes alternative routing decisions andrepeatedly determines a recommended alternative path with a preset timeinterval, with the aid of an extensive knowledge about all thealternative two-link paths. Trunk reservation is applied for all linksof the paths in the alternative routing domain.

In the document Mitra D., Gibbens R. J., Huang B. D., State-DependentRouting on Symmetric Loss Networks with Trunk Reservations-I, IEEETransactions on communications, Vol. 41 No. 2 February 1993, isdisclosed a distributed version of least loaded path routing or LLPR.The LLPR method makes the alternative routing decisions on acall-by-call basis with the aid of an extensive knowledge about thestates of the alternative two-link paths.

A distributed routing method called aggregated least busy alternativerouting or ALBA(2) is also disclosed in Mitra D., Gibbens R. J., HuangB. D., State-Dependent Routing on Symmetric Loss Networks with TrunkReservations-I, IEEE Transactions on communications, Vol. 41 No.2February 1993. In ALBA(2) the alternative routing decisions are made ona call-by-call basis with only a limited knowledge about the states ofthe alternative two-link paths. Also in this method, trunk reservationis applied for all links of the paths in the alternative routing domain.

The document EP-A1-0 229 494 discloses an isolated dynamical alternaterouting method called DAR, in which a call between two nodesinterconnected by a direct link is first offered to the direct route. Ifthe direct route is blocked, the call is offered to at currentlynominated two-link alternative route between the two nodes. If thatroute is busy, the call is lost and a randomly chosen two-link route isassigned to be the new current alternative route. Trunk reservation isapplied on alternative routes. This kind of routing method is alsoreferred to as dynamic alternative routing or DAR.

In comparison with isolated methods, centralized and distributed methodsgive a more efficient utilization of network resources, i.e. there areless blocked or lost calls for a given total number of connectingcircuits. However, the centralized and distributed methods have thedrawbacks that they entail a heavier capacity load in the nodes,complicated routing principles and great demand for information exchangebetween involved nodes and possible calculation centers.

Therefore, isolated methods are attractive but suffer from the drawbackof relatively low network efficiency.

SUMMARY OF THE INVENTION

The problem to be solved by the present invention, and thus the objectof it is to achieve an isolated alternative routing method with improvednetwork efficiency, i.e. reduced number of lost calls in a network witha given number of connecting links.

According to a first aspect of the present invention the problem issolved by setting the trunk reservation threshold for each link in analternative route depending on the position of the link. In e.g. atwo-link path, the trunk reservation threshold on the first link wouldbe set to a normal value reserving an appropriate number of circuits fordirect calls, whereas the trunk reservation threshold for the secondlink would be set lower and preferably to zero for the second link. Inthis sense the inventive method may be called unsymmetrical as itapplies trunk reservation in an unsymmetrical fashion. Consequently, ifan overflowing call has been accepted to use a first link or anattempted alternative route, it is allowed to use a second link as longas there is any idle circuit in said second link. In this way theutilization of network resources is increased, since an overflow callwhich already has occupied a first link will be accepted in moreinstances than with prior art.

According to a second aspect of the present invention an alternativepath, of which a first link has not reached its blocking threshold, isrepeatedly searched for a predefined number of times, according to apresetable sequence of alternative routes. In this text this is calledmultiple scanning, and in a preferred embodiment of the inventionmultiple scanning is applied for a first link in, for example, atwo-link path and single scanning is applied for a second link of saidpath.

In accordance with a third aspect, the present invention may compriseadaptive setting of a currently recommended route, which is the firstalternative route to be selected when a direct route is not available.In one embodiment this aspect a currently recommended route is setdependent on a currently recommended route change threshold, thusallowing the currently recommended route to be changed before a route isblocked.

The present invention also provides an apparatus for routing traffic ina circuit-switched network of nodes according to the inventive method,each node comprising at least one processor. Each processor is adaptedto offer a call between nodes via at least on preferred node and atleast on alternative route. For each processor at least one parameter isprovided, which is used to set a first and a second trunk reservationthreshold.

Advantages of the inventive method is that it requires no new inter-nodemessages and very little additional processor capacity per call. Anotheradvantage is that the rules for connections over the different links inthe considered network are very simple.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic diagram of a non-hierarchical, fullyinterconnected network with 6 nodes and direct links between each node;

FIG. 2 is a schematic diagram of the direct route and the possibletwo-link routes between the nodes 1 and 2 of the network according toFIG. 1;

FIG. 3 is a table of alternative routing domains according to FIG. 2;and

FIGS. 4 and 5 are diagrams and

FIG. 6 is a table, wherein the performance of the inventive routingmethod is compared to prior art routing methods.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows an example of a non-hierarchical, fully interconnected meshnetwork with 6 nodes, each provided with at least one processor P. Eachnode is connected to every other node by a direct link comprising anumber of two-way circuits. Calls between a pair of nodes are firstrouted via the direct link between the nodes. If the direct link isblocked, an overflowing call is routed on an alternative two-link pathvia an intermediate node.

In FIG. 2 is for example shown the direct route and the set of possibletwo-link routes between the nodes 1 and 2 of the network in FIG. 1. Moregenerally the set of possible two-link paths between a node pair i and jis called the alternative routing domain D(i,j) and is defined as thearbitrarily ordered set of two-link paths (i−>k−>j:1=<k=<N, with each kdifferent from i and j). In the present example, the network is fullyconnected and the nodes are numbered from 1 to N, with N equal to thetotal number of nodes. Thus, the alternative routing domain D(i,j) maybe regarded as a permutation p(1), p(2), . . . , p(N-2) of the numbers1,2, . . . , N except for the numbers i and j. It is also possible tochoose a subset of the possible two-link paths between two nodes as analternative routing domain. In the example of FIG. 2 D(i,j) for i=1 andj=2 is D(1,2)=(3,4,5,6).

According to an embodiment of the present invention, a currentlyrecommended alternative route or path CRP is adaptively preset as thefirst alternative route to be selected and tested for an overflowingcall. A trunk reservation threshold or a blocking threshold is set bymeans of a trunk reservation parameter for each link. For example, thenumber of circuits on each link could be 100 and the trunk reservationparameter could be set to 10. According to the invention, the trunkreservation parameter K2 for the second link in a two-link path is setto a lower value than the trunk reservation parameter K1 for the firstlink, and preferably to zero. The trunk reservation threshold for a linkis thus dependent on the direction of an alternatively routed call.

In accordance with an embodiment of the inventive method, trunkreservation is thus applied for the first link in an i−>1−>j connection,whereas a call on the second link is preferably treated as a callwithout overflow and without trunk reservation. The considered link is afirst link if the preceding node in the call attempt is located outsidethe considered mesh network and a second link if the preceding nodebelongs to said mesh network. For each originating call, the firstchoice is the direct link between the origin node and the destinationnode. If the direct link is blocked, an alternative path of which thefirst link has not reached its blocking threshold is repeatedlysearched, also called multiple scanning or overflow, for a number of upto M times according to a currently recommended routing sequence. Thesearch is started with the currently recommended alternative path CRPand is carried out sequentially or cyclically in the alternative routingdomain. If the last alternative path in the alternative routing domainis reached and rejected during a cyclical search, the first alternativepath in the alternative routing domain becomes the next to be selectedand tested.

In the first links of all M investigated alternate paths have reachedtheir blocking thresholds, the call is blocked and the next alternativepath in the alternative routing domain is set to be the currentlyrecommended path CRP for the next overflowing call.

If an alternative path not having reached its blocking threshold on itsfirst link is found within the M allowed trials and if the second isfully occupied, the call is blocked and the next alternate path in thealternative routing domain is set to be the currently recommended pathCRP for the next overflowing call.

If an alternative path not having reached its blocking threshold on itsfirst link is found within the M allowed trials and if the second linkis not fully occupied, this route is selected, the call is accepted andthe selected alternative path is set to be the currently recommendedpath CRP for the next overflowing call.

An embodiment of the, by means of an example applied for a two-linkalternative path, inventive method can be described in a more formal wayas follows. With:

D(i,j)—being the above mentioned routing domain (p(1),p(2), . . . ,p(N−2) and equal to a permutation of the numbers 1,2, . . . , N with iand j excluded;

NCRP—being the number of the intermediate node of the currentlyrecommended path CRP (i−>NCRP−>J);

M—being the maximum allowed number of multiple overflows (0<M=<N−2),i.e. multiple overflow occur when an alternatively offered route isblocked to a call and the call overflows to a second offered alternativeroute;

N—being the total number of nodes in the considered network;

X(r)—being the number of idle trunks or circuits in the first links of acurrently recommended routing sequence of M alternative pathsi−>p(c+r)−>j, for 1=<r=<M and p(c+1)=NCRP;

Y(r)—being the number of idle trunks or circuits in the second links ofa currently recommended routing sequence of M alternative pathsi−>p(c+r)−>j, for 1=<r=<M and p(c+1)=NCRP;

K(r)—being the number of reserved trunks or circuits for directly routedcalls in the first links of the currently recommended routing sequenceof M alternative paths i−>p(c+r)−>j, for 1=<r=<M and p(c+1)=NCRP; and

m—denoting the first of the M alternative paths of a currentlyrecommended routing sequence fulfilling the condition X(m)>K(m), i.e.m=min (1=<r=<M: X(r)>K(r), and if there is no such r, 1=<r=<M,fulfilling this requirement, then setting m>M.

With these definitions the following steps are comprised in anembodiment of the inventive method:

1) A call between an origin-destination node pair i and j is firstoffered to a direct link i−>j. If said direct link i−>j is not fullyoccupied, the call is accepted on this link and the currentlyrecommended path CRP is kept, otherwise the call is further treatedaccording to step 2, see below.

2) The call is offered to the alternative paths in the routing domainD(i,j) of said node pair according to a currently recommended routingsequence of M alternative paths i−>p(c+r)−>j, for 1=<r=<M andp(c+1)=NCRP.

If m>M, i.e. if X(r)=<K(r) for all alternative paths i−>p(c+r)−>j, for1=<r=<M and p(c+1)=NCRP, the call is blocked and the next alternativepath in the alternative routing domain D(i,j) is set to be the currentlyrecommended path CRP for the next overflowing call from the direct linki−>j. If m=<M, the call is treated according to step 3, see below.

3) If m=min (1=<r=M: X(r)>K(r), m=<M and Y(m)=0, the call is blocked andthe alternative path i−>p(c+M+1)−>j is set to be the currentlyrecommended path CRP for the next overflowing call from the direct linki−>j.

If m=<M and Y(m)>0, the call is accepted and the alternative pathi−>p(c÷m)−>j is kept as the currently recommended path CRP for the nextoverflowing call from the direct link i−>j.

FIG. 3 shows tables I, II and III illustrating the alternative routingdomain and pointers used to set the currently recommended path CRP inanother example of a mesh network comprising e.g. 13 nodes, and animplementing principle of an embodiment of the invention. Table I showsthe routing domain D(i,j) (r) for 1=<r=<M, M=N−2 ordered in a currentlyrecommended routing sequence, table II shows the corresponding number kof intermediate nodes in said sequence and shows the states for numberof circuits, number of busy circuits and trunk reservation parameter ofthe first links i−>k in the two-link paths of said sequence. In thisinstance a first pointer is set on CRP=7, and a second pointer is set onthe state of first line i=>k, k=7 in the state table III.

In a further embodiment of the invention the trunk reservationparameters for first links are periodically or continuously adapted tothe actually observed traffic on the network. The trunk reservationparameters may in this instance be non-negative integer valuedstochastic processes with its values determined by the originating nodesbased on traffic observations at these nodes.

Overflow calls have a tendency to occur closely in time, that is in anon-stochastic way or according to a non-Poissonian behaviour. For thisreason currently recommended alternative paths may be protected fromoverflowing, e.g. by changing the currently recommended route or pathCRP, if an overflowing call from a particular direct link is followed byanother overflow call from the same direct link within a specified andpresetable time limit.

Test Results

The test results shown that the inventive method have better performancethan prior art isolated methods and performance close to much moreresource demanding centralized and distributed methods, when the networksystems are lightly loaded and have a small number of circuits in thelinks. As the number of circuits increases towards infinity, all methodsasymptotically become equivalent.

In FIGS. 4 and 5 is compared, by means of simulation results, theperformance of seven methods, whereof 5 prior art methods and twoembodiments of the inventive method, as functions of the link traffic A,when implemented in a fairly small system with 12 nodes and links ofequal size with 120 circuits. The compared methods are: 1. Only directtraffic; 2. Dynamical alternative routing—DAR; 3. The inventivemethod—UDR; 4. The inventive method with adaptive setting of thecurrently recommended route UDR-CT; 5. ALEA(2); 6. CLLPR; and 7. LLPR.In FIG. 4 all direct traffic is equal to all links and in FIG. 5 thereis an unbalanced traffic situation, with the outgoing link traffic equalto /A÷0.1A)/2 for six nodes and equal to (A−0.1A) for the other sixnodes.

The situation shown in FIG. 5, where one part of the network is heavilyloaded while the other part is lightly loaded, can e.g. Illustrate anetwork that meets different traffic situations during different timesof day. Examples of this case is an interstate network coveringdifferent time-zones, or a network serving a city and its suburbs. Inthe latter example there is heavy traffic in the central part of thecity during office hours, and in the evening the traffic load is shiftedto the suburbs. A third example is the case with wrongly forecastedtraffic patterns in a network.

What is claimed is:
 1. A method for routing traffic in acircuit-switched network comprising: performing at plural nodes of thecircuit-switched network the steps of: offering a call between anoriginating node and a destination node as a preferred route betweensaid nodes; if said preferred route is not available, offering a firstalternative route via at least one intermediate node, thus creating afirst alternative route including at least two links; setting a firsttrunk reservation threshold for reserving a certain number of circuitsfor direct calls along a first link between the originating node and theat least one intermediate node; and setting a second trunk reservationthreshold for a second link between the at least one intermediate nodeand a next node in the first alternative route; said second trunkreservation threshold being set lower than the first trunk reservationthreshold.
 2. A method as recited in claim 1, wherein said second trunkreservation threshold is set to zero for making said second linkavailable in the presence of any idle circuit in said second link.
 3. Amethod as recited in claim 1, said method further comprising: selectingand investigating a second alternative route when a number of idlecircuits on the first link of the first offered alternative route isless than said first trunk reservation threshold, where a call is lostif said second alternative route is not available.
 4. A method asrecited in claim 1, further comprising: selecting and investigating apresetable number of second alternative routes, where a call is lost ifnone of said second alternative routes is available.
 5. A method asrecited in claim 1, further comprising: selecting a presetable currentlyrecommended route as the first offered alternative route, and selectingand investigating a presetable number of alternative routes in sequenceuntil either an available route is found or the call is lost if saidcurrently recommended route is not available.
 6. A method as recited inclaim 5, further comprising: setting the currently recommended route tothe next alternative route in said sequence if the number of idlecircuits on the first link of the first offered alternative route isless than said first trunk reservation threshold and none of thealternative routes in said sequence is available.
 7. A method as recitedin claim 5, further comprising: setting the currently recommended routeto a second alternative route next to said first alternative route ifthe number of idle circuits on the first link of the second offeredalternative route in said sequence is larger than said first trunkreservation threshold and there is no idle circuit on the second link.8. A method as recited in claim 5, further comprising: setting acurrently recommended route change threshold, and changing saidcurrently recommended route based on said currently recommended routechange threshold.
 9. A method as recited in claim 5 further comprising:changing said currently recommended route if an overflow call from aparticular direct link is followed by another overflow call from thesame direct link within a specified and presetable time limit.
 10. Amethod as recited in claim 1, wherein said preferred route is a singlelink route and said alternative route is a two-link route.
 11. A methodas recited in claim 1, further comprising: periodically or continuouslyadapting said first trunk reservation threshold to observed traffic,wherein the trunk reservation threshold may be a non-negativeinteger-valued stochastic process with values for the process beingdetermined by the originating nodes based on traffic observations at theoriginating nodes.
 12. The method of claim 1 wherein the next node isone of an intermediate node and a destination node.
 13. A method forrouting traffic in a circuit-switched network comprising: offering acall between an original node and a destination node as a preferredroute between said nodes: offering a first alternative route via anintermediate node if said preferred route is not available; setting afirst trunk reservation threshold for a first link between the originalnode and a first node, said first trunk reservation thresholddesignating a certain number of circuits to be reserved for direct callsalong said first link between the original node and the first node, andsetting a second trunk reservation threshold for calls between the firstnode and the second node connecting the second link of said alternativeroute; wherein said step of offering a first alternative route includes,selecting a presetable currently recommended route as the first offeredalternative route; said method selecting and investigating a presetablenumber of alternative routes as said first alternative route in sequenceuntil either an available route is found or the call is lost, if saidcurrently recommended route is not available; said method furthercomprising; selecting the alternative route with a lowest number in thesequence of said presettable number of alternative routes for which anumber of idle circuits on said first link thereof is larger than thefirst trunk reservation threshold and for which there is no idle circuiton said second link if none of the alternative routes of said sequenceis available, and setting the currently recommended route to saidselected alternative route.
 14. A method for routing traffic in acircuit-switched network comprising: offering a call between an originalnode and a destination node as a preferred route between said nodes;offering a first alternative route via an intermediate node is saidpreferred route is not available; setting a first trunk reservationthreshold for a first link between the original node and a first node,said first trunk reservation threshold designating a certain number ofcircuits to be reserved for direct calls along said first link betweenthe original node and the first node, and setting a second trunkreservation threshold for calls between the first node and the secondnode connecting the second link of said alternative route; wherein saidstep of offering a first alternative route includes, selecting apresetable currently recommended route as the first offered alternativeroute; said method selecting and investigating a presetable number ofalternative routes as said first alternative route in sequence untileither an available route is found or the call is lost, if saidcurrently recommended route is not available; said method furthercomprising; selecting the alternative route with a lowest number in thesequence of said presettable number of alternative routes for which anumber of side circuits on said first link thereof is larger than thefirst trunk reservation threshold and for which there is no idle circuiton said second link if none of the alternative routes of said sequenceis available, and accepting the call, said step of setting the currentlyrecommended route to said selected alternative route being performedafter said step of accepting the call.
 15. A method for routing trafficin a circuit-switched network comprising: offering a call between anoriginal node and a destination node as a preferred route between saidnodes; offering an first alternative route via an intermediate node ifsaid preferred route is not available; setting a first trunk reservationthreshold for a first link between the original node and a first node,said first trunk reservation threshold designating a certain number ofcircuits to be reserved for direct calls along said first link betweenthe original node and the first node, and setting a second trunkreservation threshold for calls between the first node and the secondnode connecting the second link of said alternative route; wherein saidstep of offering a first alternative route includes, selecting apresetable currently recommended route as the first offered alternativeroute; said method selecting and investigating a presetable number ofalternative routes as said first alternative route in sequence untileither an available route is found or the call is lost, if saidcurrently recommended route is not available; said method furthercomprising; changing said currently recommended route based on saidcurrently recommended route change threshold, and setting a currentlyrecommended route change threshold for said second link of the firstalternative route, and selecting a new currently recommended route forthe traffic from said originating node to said destination node if anoverflowing call is accepted on said selected first alternative routeand if the number of idle circuits on said second link is less or equalto the sum of the trunk reservation threshold.
 16. An apparatus ofrouting traffic through a circuit-switched network, comprising: at leastone processor at each node of the network, plural of said processors atplural nodes being structured and arranged including: means for offeringa preferred route for calls between an origin node and a destinationnode, means for offering an alternative route between the origin anddestination nodes via at least one intermediate node, if said preferredroute is not available, means for setting a first trunk reservationthreshold parameter for reserving a certain number of direct calls alonga link between said originating node and the at least one intermediatenode, and means for setting a second trunk reservation thresholdparameter for calls between said at least one intermediate node and anext node in said alternative route.
 17. An apparatus as recited inclaim 16, wherein the link between the originating node and said atleast one intermediate node is a first link and the link between the atleast one intermediate node and a next node is a second link; whereinthe means for setting the second trunk reservation threshold parameterset said second trunk reservation threshold to zero for making saidsecond link available in the presence of any idle circuit in said secondlink.
 18. An apparatus as recited in claim 16, further comprising: meansfor selecting and investigating a second alternative route, where thecall is lost if said second alternative route is not available.
 19. Anapparatus as recited in claim 16, further comprising: means forselecting and investigating a presetable number of second alternativeroutes, if none of said second alternative routes is available.
 20. Anapparatus as recited in claim 16, further comprising: means forselecting a presetable currently recommended route as a first offeredalternative route, and means for selecting and investigating apresetable number of alternative routes in sequence until either anavailable route is found or the call is lost.
 21. An apparatus asrecited in claim 20, further comprising: means for setting the currentlyrecommended route to the next alternative route in said sequence if thenumber of idle circuits on the first link of an offered alternativeroute is less than said first trunk reservation threshold and thus noneof the alternative routes in said sequence is available.
 22. Anapparatus as recited in claim 20, wherein the link between theoriginating node and said at least one intermediate node is a first linkand the link between the at least one intermediate node and a next nodeis a second link; said apparatus further comprising: means for settingthe currently recommended route to the alternative route next to saidoffered alternative route if the number of idle circuits on the firstlink of an offered alternative route in said sequence is larger thansaid first trunk reservation threshold and there is no idle circuit onthe second link.
 23. An apparatus as recited in claim 20, furthercomprising: means for setting a currently recommended route changethreshold, and means for changing said currently recommended route basedon said currently recommended route change threshold.
 24. An apparatusas recited in claim 20, wherein said preferred route is a single linkroute and said alternative route or routes are two-link routes.
 25. Anapparatus as recited in claim 20, further comprising: means forperiodically or continuously adapting said first trunk reservationthreshold to the actually observed traffic, wherein the trunkreservation threshold may be determined by non-negative integer valuedstochastic processes with values for the process being determined by theoriginating nodes based on traffic observations at the originatingnodes.
 26. An apparatus as recited in claim 20, further comprising:means for changing said currently recommended route if an overflow callfrom a particular direct link is followed by another overflow call fromthe same direct link within a specified and presetable time limit. 27.The apparatus of claim 16 wherein the next node is one of anintermediate node and a destination node.
 28. An apparatus for routingtraffic through a circuit-switched network comprising: at least oneprocessor at each node of the network, said processors being structuredand arranged including: means for offering a preferred route for callsbetween an origin node and a destination node; means for offering analternative route between the origin and destination nodes via anintermediate node if said preferred route is not available; means forsetting a first trunk reservation threshold parameter for reserving acertain number of direct calls along said link between said origin nodeand a first intermediate node; and means for setting a second trunkreservation threshold parameter for calls between said firstintermediate node and a next node connecting a second link of saidalternative route; said means for offering a preferred route including,means for selecting a presetable currently recommended route as a firstoffered alternative route; means for selecting and investigating apresetable number of alternative routes in sequence until either anavailable route is found or the call is lost; means for selecting thealternative route with a lowest number of said sequence for which anumber of idle circuits on said first link is larger than the trunkreservation threshold for said first link and for which there is atleast one idle circuit on said second link if one or more alternativeroutes of said sequence is available, and means for setting thecurrently recommended route to said selected alternative route.
 29. Anapparatus for routing traffic through a circuit-switched networkcomprising: at least one processor at each node of the network, saidprocessors being structured and arranged including: means for offering apreferred route for calls between an origin node and a destination node;means for offering an alternative route between the origin anddestination nodes via an intermediate node if said preferred route isnot available; means for setting a first trunk reservation thresholdparameter for reserving a certain number of direct calls along said linkbetween said origin node and a first intermediate node; and means forsetting a second trunk reservation threshold parameter for calls betweensaid first intermediate node and a next node connecting a second link ofsaid alternative route; said means for offering a preferred routeincluding, means for selecting a presetable currently recommended routeas a first offered alternative route; means for selecting andinvestigating a presetable number of alternative routes in sequenceuntil either an available route is found or the call is lost; means forselecting the alternative route with a lowest number in said sequencefor which a number of idle circuits on said first link is larger thanthe trunk reservation threshold for said first link and for which thereis at least one idle circuit on said second link if one or morealternative routes of said sequence is available, means for acceptingthe call, and means for setting the currently recommended route to saidselected alternative route.
 30. An apparatus for routing traffic througha circuit-switched network, comprising: at least one processor at eachnode of the network, said processors being structured and arrangedincluding: means for offering a preferred route for calls between anorigin node and a destination node; means for offering an alternativeroute between the origin and destination nodes via an intermediate nodeif said preferred route is not available; means for setting a firsttrunk reservation threshold parameter for reserving a certain number ofdirect calls along said link between said original and intermediatenodes; and means for setting a second trunk reservation thresholdparameter for calls between said intermediate node and a next nodeconnecting a second link of said alternative route; said means foroffering a preferred route including, means for selecting a presetablecurrently recommended route as a first offered alternative route; meansfor selecting and investigating a presetable number of alternativeroutes in sequence until either an available route is found or the callis lost; means for setting a currently recommended route changethreshold; means for changing said currently recommended route based onsaid currently recommended route change threshold; means for setting acurrently recommended route change threshold for said second link of analternative route, and means for selecting a new currently recommendedroute for the traffic from said originating node to said destinationnode if an overflowing call is accepted on said alternative route and ifthe number of idle circuits on said second link is less or equal to thesum of the trunk reservation threshold.